This invention relates to measuring the concentration of ions implanted in semiconductor materials.
Most microelectronic devices (e.g., microprocessors) include a series of thin insulating (e.g. oxide) and conducting (e.g., metal and polysilicon) films grown or deposited on a single-crystal silicon wafer. Both silicon wafers and polysilicon films are implanted with ions during fabrication so that they exhibit a specified electrical conductance. For example, the silicon may be implanted to create regions that function as a "p" or "n" type semiconductor. After implantation, both silicon and polysilicon are annealed under high temperatures to heal lattice damage that typically results from the implantation process.
The electrical conductance of silicon and polysilicon is strongly affected by three properties of the implanted ions: (1) atomic composition; (2) implantation energy; and (3) ion concentration or dose. Silicon's electrical conductivity is particularly dependent on the concentration of the implanted ions. For example, silicon wafers are typically implanted with arsenic, argon, phosphorous, oxygen, or boron ions ranging in concentration from 10.sup.12 -10.sup.16 cm.sup.-3. These ions are typically implanted with energies ranging from a few keV to thousands of keV.
The performance of a completed microelectronic device depends critically on the electrical properties of the silicon wafer and the overlying polysilicon films, and thus the properties of ions implanted in these materials are carefully monitored during fabrication. One property, called electrical sheet resistance, is measured by contacting a sample's surface with an electrical-testing instrument called a 4-point probe. Electrical current flowing from one probe to another depends on the resistance of the material. Resistance, in turn, varies inversely with the concentration of implanted ions.
Ion concentration is also monitored using a non-contact, optical method that excites and detects both a photothermal response and an electron-hole plasma within the silicon. To make this measurement, a first laser beam irradiates the silicon and is partially absorbed to generate either (or both) the electron-hole plasma and photothermal response. These responses modify the reflectivity of the sample's surface and are measured with a second laser beam. This is partially reflected by the sample and then analyzed to estimate the concentration of the implanted ions.
Although used throughout the microelectronics industry to determine the concentration of implanted ions, both 4-point probes and instruments that measure optical reflectivity suffer disadvantages. 4-point probes necessarily contact the sample, and are therefore destructive. This means that these instruments can only measure "monitor" wafers or regions of "product" wafers that lack functioning devices. In addition, 4-point probes can only measure annealed wafers. Optical-reflectivity instruments have a limited scope of measurement and generate signals that are often difficult to interpret: they are therefore used primarily to determine whether or not a sample has been ion implanted with ions, rather than the actual concentration of the implanted ions.